I have had tmj and muscle tightness. what should it feel like when muscles are loosening up . i think this is happening.

The pain will lessen: The pain will lessen and range of motion will increase.

Good sign: Loosening of the joint/joints and the muscles could be good signs. But don't get too over confident too soon and create a setback. Still focus on no wide yawns, smaller bites and whatever therapy was recommended. Tmj/tmd can easily recycle and come back, sometimes worse.

No soreness: Palpation of the facial muscles will not illicit soreness. There should be a lack of trigger points and the face should look relaxed and no clenching.

Improved symptoms: If the muscles are loosening, the "tightness" you describe will decrease.